1. Field of the Invention
The present invention relates to an idler mechanism for a magnetic tape device, particularly to an idler mechanism for a magnetic tape device in which an idler gear is swingingly displaced by using a frictional resistance.
2. Description of the Related Art
A tape device such as a magnetic recording and reproducing device has a pair of reels to which a supply tape reel and take-up tape reel housed in a tape cassette are respectively attached. The reels rotate the tape reels in a forward direction and a reverse direction. An idler mechanism is used for switching over the rotational direction of the reels.
FIG. 3 is a partially cutaway schematic side view of a conventional idler mechanism. In this type, an idler gear is swingingly displaced between a first position where rotation is transmitted to one of reels, and a second position where rotation is transmitted to the other reel.
In FIG. 3, 1 denotes an idler gear, and 2 denotes an input gear which always meshes with the idler gear 1 and is rotated in forward and reverse directions. An arm-like swinging member 3 is disposed on the gears 1 and 2. One end of the swinging member 3 is coupled in a relatively rotatable manner to a center shaft 21 of the input gear 2. A shaft hole 31 is formed in the other end of the swinging member 3. The idler gear 1 is rotatably attached to a shaft 4 which is inserted into the shaft hole 31. A spring 5 for urging the idler gear 1 toward the swinging member 3 is interposed between a spring bracket 41 disposed on the shaft 4 and the idler gear 1.
In the idler mechanism, when the input gear 2 is rotated, the idler gear 1 is rotated in the direction which is opposite to the rotational direction of the input gear 2. Since the idler gear 1 is urged toward the swinging member 3 by the spring 5, a frictional resistance is generated in an overlapping region “a” between the idler gear 1 and the swinging member 3. In a meshing region “b” between the input gear 2 and the idler gear 1, therefore, a rotational torque which causes the idler gear 1 to swing about the center shaft 21 of the input gear 2 is generated. The idler gear 1 is swingingly displaced about the center shaft 21 by the rotational torque. When the rotational direction of the input gear 2 is switched over to the forward or reverse direction, the idler gear 1 is swingingly displaced between a first position where rotation is transmitted to one of reels, and a second position where rotation is transmitted to the other reel.
JP-A-57-100646 discloses an idler mechanism shown in FIGS. 4 and 5. An idler gear is swingingly displaced between first and second positions without using the spring 5 shown in FIG. 3.
The idler mechanism shown in FIGS. 4 and 5 is used for driving reels of a tape recorder. A center portion of a support lever 93 is coupled in a relatively rotatable manner to a capstan shaft 90 to which a capstan gear 91 is fixed. A pair of idler gears 94 and 95, which always mesh with the capstan gear 91, are rotatably supported by shafts 96 and 97 that are attached to both ends of the support lever 93 respectively. The lower ends of the shafts 96 and 97 are placed on the upper face of a flywheel 98 which is fixed to the capstan shaft 90.
When the capstan gear 91 is rotated, the capstan shaft 90 and the flywheel 98 are rotated together, and hence the shafts 96 and 97 which are placed on the upper face of the flywheel 98 are displaced together with the idler gears 94 and 95 about the capstan shaft 90 by the flywheel 98. When the rotational direction of the capstan gear 91 is switched over to the forward or reverse direction, the idler gears 94 and 95 are swingingly displaced between a first position where rotation is transmitted to one of reels, and a second position where rotation is transmitted to the other reel. In the first position, the idler gear 94 meshes with a reel gear 92 which rotates the one reel 92a. In the second position, the idler gear 95 meshes with a reel gear 99 which rotates the other reel 99a. 
In the conventional structure described with reference to FIG. 3, a frictional resistance is generated by urging the idler gear 1 toward the swinging member 3 by the spring 5, and a rotational torque which causes the idler gear 1 to swing is generated by the frictional resistance. Therefore, the spring 5 is inevitably used for swingingly displacing the idler gear 1 between the first and second positions. This causes problems in that the number of parts is increased, that the assembly process becomes cumbersome, and that the cost is raised. As shown in FIG. 3, the shaft 4 is additionally required for attaching the idler gear 1 and the swinging member 3, thereby causing a further problem in that the cost is correspondingly increased.
By contrast, in the configuration described with reference to FIGS. 4 and 5, no spring is used, and hence the number of parts can be correspondingly reduced. In the configuration, however, under the state where the idler gears are located in the first or second position to mesh with the reel gear 92 or 99, the upper face of the flywheel 98 rubs with the lower ends of the shafts 96 and 97 because the shafts 96 and 97 are held to given positions but the flywheel 98 is rotated together with the capstan shaft 90. Therefore, the lower ends of the shafts 96 and 97, or the upper face of the flywheel 98 easily wears, and there arises the possibility that the performance stability of the idler mechanism is impaired by the wear.